1. Field of the Invention
The invention relates to a control apparatus for a vehicle which is capable of stopping the supply of fuel to an internal combustion engine during deceleration.
2. Description of the Related Art
Generally, a vehicle equipped with an internal combustion engine is designed to perform a fuel-cut process of stopping the supply of fuel to the internal combustion engine so as to improve fuel economy.
Such a vehicle is designed so that after an accelerator pedal of the vehicle is released and the vehicle begins to decelerate, the fuel-cut control of stopping the supply of fuel to the internal combustion engine is executed until the engine rotation speed of the internal combustion engine drops to a predetermined value. Then, after the engine rotation speed drops to the predetermined value, the vehicle ends the execution of the fuel-cut and starts supplying fuel again in order to prevent occurrence of engine stall.
Among vehicle control apparatuses that execute the fuel-cut control, there is known a control apparatus that switches between the execution and the non-execution of the fuel-cut according to the temperature of a catalyst that is disposed in an exhaust passageway of the internal combustion engine, so as to prevent degradation of the catalyst (see, e.g., Japanese Patent Application Publication No. 11-62664 (JP-A-11-62664)).
The vehicle control apparatus described in Japanese Patent Application Publication No 11-62664 (JP-A-11-62664) is designed to execute the fuel-cut control when the degree of throttle opening is a fully closed degree and the rotation speed of the internal combustion engine is greater than or equal to a predetermined value.
It is to be noted herein that if a fuel-lean exhaust gas passes through the catalyst in a high temperature state, the excess supply of oxygen accelerates the growth of particles of platinum (Pt) in the catalyst. Therefore, the total surface area of Pt that contacts exhaust gas in the catalyst reduces, so that the exhaust gas control performance declines.
Therefore, the vehicle control apparatus described in Japanese Patent Application Publication No. 11-62664 (JP-A-11-62664) determines whether or not the catalyst is in a high temperature state during execution of the fuel-cut control. When it is determined that the catalyst is in the high temperature state, the control apparatus prohibits execution of the fuel-cut control even if the condition for executing the fuel-cut control is satisfied. In this manner, the control apparatus prevents the catalyst from being exposed to lean exhaust gas and therefore prevents reduction of the surface area of Pt in the catalyst, and thus maintains the exhaust emission control performance.
On another hand, if the fuel-cut control is prohibited during the high temperature state of the catalyst as described above while the internal combustion engine is warming up, misfire may occur due to unstable combustion resulting from high viscosity of the lubricating oil of the engine or low atomization of fuel. If unburnt fuel is supplied together with exhaust gas into the catalyst, and ignites therein, the catalyst performance is rather likely to degrade. Therefore, the vehicle control apparatus described in Japanese Patent Application Publication No. 11-62664 (JP-A-11-62664) discontinues the prohibition of the fuel-cut control during the warming-up of the internal combustion engine, even if the catalyst is in the high temperature state.
Beside, generally in conjunction with the vehicle control apparatuses that execute the fuel-cut control as described above, since combustion is unstable during the warming-up state of the internal combustion engine (during a state in which the engine is being warmed up), the likelihood of occurrence of engine stall heightens if the fuel-cut control is ended while the engine rotation speed has become low. Therefore, during the warming-up state, the engine rotation speed at which to end the fuel-cut control is set higher than after the end of the warming-up, so as to prevent occurrence of engine stall.
Therefore, when the engine is being warmed up, the continuation time of the fuel-cut becomes shorter and therefore improvement of the fuel economy is hampered, in comparison with when the engine has been warmed up. Therefore, in order to realize improved fuel economy, it is advisable to delay the end of the fuel-cut control as much as possible by adopting a construction in which the engine rotation speed at which to end the fuel-cut control during the warming-up of the engine is set lower than in the related art.
However, the related-art vehicle control apparatus described in Japanese Patent Application Publication No. 11-62664 (JP-A-11-62664) is not designed so that the engine rotation speed at which to end the fuel-cut control during the warming-up of the engine is set lower than in the related art. Therefore, if the fuel-cut control during the warming-up of the engine is executed at lower engine rotation speeds than in the related art, it is likely that when the supply of fuel is started again, the combustion will be unstable and the engine rotation speed will not rise, and therefore a drawback, such as engine stall or the like, will occur.